ARM: sun8i: v3s: enable SPI

Allwinner V3s SoC has a SPI controller, muxed with the MMC2 controller
at PC bank. The controller itself is identical to the one in H3 SoC.

Add device tree node and the only pinmux node for it.

Tested with a Winbond W25Q128FV SPI NOR soldered on the Lichee Pi
early sample.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>